All arithmetic is performed modulo 2^n.
One non-obvious consequence of this is that negate
should not raise an error on negative arguments.

For coercing between any two integer types, use
fromIntegral, which is specialized for all the
common cases so should be fast enough. Coercing word types to and
from integer types preserves representation, not sign.

It would be very natural to add a type a type
Natural providing an unbounded size unsigned
integer—just as Integer provides unbounded
size signed integers. We do not do that yet since there is no demand
for it. Doing so would require Bits.bitSize to
return Maybe Int.

The rules that hold for Enum instances over a bounded type
such as Int (see the section of the Haskell report dealing
with arithmetic sequences) also hold for the Enum instances
over the various Word types defined here.

Right and left shifts by amounts greater than or equal to the width of
the type result in a zero result. This is contrary to the behaviour
in C, which is undefined; a common interpretation is to truncate
the shift count to the width of the type, for example 1 <<
32 == 1 in some C implementations.